Cladding-pumped fiber devices, such as lasers, amplifiers and light combiners, are important in a wide variety of optical applications, including high power communication systems, light sources for printers, lasers for medical optics, and the like. A typical cladding-pumped optical fiber comprises a signal core and a plurality of cladding layers. The inner cladding surrounding the core is typically a silica cladding of large cross-sectional area (as compared to the core) and high numerical aperture (NA). It is usually non-circular to ensure that the modes of the inner cladding will exhibit good overlap with the core. An outer coating is commonly composed of a low index polymer. The index of the core is greater than that of the inner cladding which, in turn, is greater than the index of the outer coating.
A major advantage of the cladding-pumped fiber is that it can convert light from low brightness sources into light of high brightness in the single mode fiber core. Light from low brightness sources, such as diode arrays, can be coupled into the inner cladding as a result of its large cross-sectional area and high numerical aperture. In a cladding-pumped laser or amplifier, the core is doped with a rare earth such as ytterbium (Yb) or erbium (Er). The light in the cladding interacts with the core and is absorbed by the rare earth dopant. If an optical signal is passed through the pumped core, it will be amplified. Alternatively, if optical feedback is provided (as with a Bragg grating optical cavity), the cladding-pumped fiber will act as a laser oscillator at the feedback wavelength.
FIG. 1 illustrates an exemplary prior art cladding-pumped fiber 1 having a core 2, an inner (or pump) multimode cladding layer 3, and an outer coating 4. Inner cladding layer 3 exhibits a refractive index lower than that of core 2 such that the light signal L propagating along core 2 will remain confined therein, as shown in FIG. 1. Similarly, outer coating 4 confines pumping light P within the boundaries of inner cladding layer 3, as shown. In accordance with the cladding-pumped arrangement, the rays comprising pump light P periodically intersect core 2 for absorption by the active material therein, so as to generate or amplify light signal L. It is to be noted that since inner cladding 3 is multimode, many rays other than those shown by the arrows in FIG. 1 can propagate within inner cladding 3.
A difficulty preventing full exploitation of the potential of cladding-pumped fiber devices is the problem of efficiently coupling a sufficient number of low brightness sources into the inner cladding. A proposed solution to this problem is described in U.S. Pat. No. 5,864,644, entitled “Tapered Fiber Bundles for Coupling Light Into and Out of Cladding-Pumped Fiber Devices”, issued to D. J. DiGiovanni et al. on Jan. 26, 1999. In the DiGiovanni et al. arrangement, light is coupled from a plurality of sources to a cladding-pumped fiber by the use of a tapered fiber bundle, formed by grouping individual fibers into a close-packed formation and heating the collected fibers to a temperature at which the bundle can be drawn down into a tapered configuration. The taper is then fusion spliced to the cladding-pumped fiber. FIG. 2 illustrates an exemplary embodiment of this DiGiovanni et al. prior art approach, where a plurality of pump fibers 5 are shown as distributed around a fiber containing a core 6. As shown, the entire bundle 7 is fused and tapered along a section 8 to a single output cladding-pumped fiber 9. As described therein, tapering of the fiber bundle is performed to increase the intensity of pump light coupled into the end of cladding-pumped fiber 9. Inasmuch as the NA of the multimode pump region is much greater than the NA of the pump fibers, tapering of the fiber bundle allows for an increase in the optical pump intensity while remaining within the angular acceptance of the multimode pump region.
Even though the DiGiovanni et al. tapered fiber bundle has been found to greatly improve the efficiency of coupling multiple optical signals into a fiber amplifier, laser or light combiner, problems attributed to the presence of “stray light” within the system remain to be solved. Stray light has been found to arise from a number of different sources, such as amplified spontaneous emission (ASE) within a gain fiber, unabsorbed or scattered pump light, and signal light that has scattered out of the core and into the inner cladding. While the prior art arrangement of FIG. 1 is capable of transmitting stray light with minimal attenuation and without heating the fiber, stray light may result in catastrophic heating if it is not permanently contained within the boundary of inner cladding 3. The escape of stray light from the cladding can occur if the NA of the cladding light is increased at a perturbation (such as a taper) to exceed the NA between inner cladding 3 and outer coating 4. In this situation, cladding light refracts into outer coating 4 where it is absorbed and generates an unwanted amount of localized heating. Stray light may also refract into outer coating 4 at a termination of the cladding-pumped fiber, such as at the point where it is spliced to an output fiber (such fibers generally have a high index outer coating) or at any point along the fiber where it is bent to a degree sufficient to couple light into the cladding layer.
FIG. 3 illustrates the above-described situation where stray light is associated with a termination condition, in this case at a splice S between cladding-pumped fiber 1 of FIG. 1 and an output fiber 11. As shown, unabsorbed/scattered pump light remaining at the termination of cladding-pumped fiber 1 enters output fiber 11 and refracts into a high index polymer outer coating 13. Since the optical absorption of polymer outer coating 13 is much greater than that of glass, a significant portion of the light is absorbed by coating 13 and converted to heat. If this heat is sufficiently localized, the fiber may be burned or otherwise damaged to the point of experiencing catastrophic failure. Besides the presence of unabsorbed pump light, signal light can also be scattered out of core region 2 at the termination of fiber 1, whereupon it will propagate along inner cladding 15 and may also refract into high index cladding 13 to cause additional heating.
While heating can arise at a splice location between two dissimilar fibers (as shown here in FIG. 3), splices between identical fibers may also generate heat, as a result of slight imperfections that cause light scattering. Various other types of perturbations along the fiber may also result in increasing the presence of stray light along the fiber and thus potentially compound the problem of locally heating the fiber. Since the optical power levels can be high in amplifier applications, it is best to gradually dissipate the energy, thus avoiding localized heating of the fiber or any of its associated optical components.
Prior art attempts to address this problem typically involve the use of sections of “absorbing” fiber interspersed along the transmission path, where these sections include selectively absorbing species, such as rare earth ions, in concentrations sufficient to provide the desired absorbance selectivity. U.S. Pat. No. 6,574,406 issued to B. J. Ainslie et al. on Jun. 3, 2004, and US Application 2004/0175086 by L. A. Reith et al. and published on Sep. 9, 2004, disclose two different arrangements of this principle.
While these arrangements provide a certain degree of stray light management, the utilization of selected sections of fiber to provide this ability limits its usefulness. For example, if a new splice is added to a fiber, or a bend is introduced in a new location, the absorbing fiber sections may not be properly located to dissipate additional stray light. Moreover, the fiber section dimensions need to be carefully controlled to ensure that the energy is dissipated in a sufficiently gradual manner.
Thus, a need remains in the art for a configuration that is capable of managing the presence of stray light within an optical fiber so as to minimize heating of the fiber and/or other failure modes attributed to the presence of stray light.